An object information acquiring apparatus (photoacoustic apparatus) which was developed for use in investigating breast cancer has been described in Non Patent Literature 1. In Non Patent Literature 1, an object (breast) is compressed between a glass plate and an ultrasound probe, and illumination light (near-infrared light) using an Nd: YAG laser as a light source is irradiated onto the breast through the glass plate. The photoacoustic wave generated inside the object (breast) is received by the ultrasound probe, and the internal tissue of the object (breast), and in particular, the generation of new blood vessels in breast cancer, is reconstructed as an image and displayed. When image reconstruction is carried out in this way, the speed of sound in the object is used in calculation. In general, the average speed of sound in a breast is considered to be 1510 m/s. Using photoacoustic wave in this way to reconstruct an image is called PAT (Photoacoustic tomography).
However, in the apparatus in Non Patent Literature 1, there is no mention of correcting change in the speed of sound of the object (breast). Therefore, if the supposed speed of sound is different from the speed of sound in the object, then this gives rise to decline in resolution. Therefore, desirably, change in the speed of sound is corrected. Patent Literature 1 is one method of this kind.
FIG. 5 shows an approximate view of Patent Literature 1, in which a holding plate 502 which transmits a ultrasound wave and a compression plate 503 which reflects the ultrasound wave are provided, and a probe 501 transmits an ultrasound beam onto an inspection object 500. An echo signal which is transmitted through the holding plate 502 and is reflected by the compression plate 503 is received by the probe 501 and recorded. A unit is provided which extracts the propagation time and/or amplitude from this echo signal, and evaluates same by comparison with a propagation time and/or amplitude of a reference echo signal. In this case, it is possible to calculate an average speed of sound on the basis of the propagation time and the interval between the compression plate 503 and the holding plate 502.